1. Field of the Invention
The present invention relates to antistatic additives for chemical compositions, and, more particularly, to antistatic hexahalogenated ionic compounds, and antistatic additives including hexahalogenated ionic compounds for organic polymer compositions such as polyurethane foams.
2. Description of the Related Art
a. Uses of Organic Polymer Compositions:
Flexible polyurethane (PU) foam is used widely as a cushioning material in packaging applications. PU foam, however, is an inherently static-producing material. PU foam must be rendered "anti-static" in order to package static-sensitive items, such as computer disks, computer chips, and electronic components. Various additives can alter the electrostatic properties of the foam so that it becomes static-dissipative, and therefore useful as a material with protection against static discharge.
The ease of cutting and fabricating flexible antistatic PU foam into complex shapes makes it an ideal cushioning material for composite packaging for many static sensitive products, as noted above.
In addition, antistatic PU foam in sheet form also competes with various types of polyolefin wrappings (antistatic blown polyethylene sheeting) in static-protected lined envelopes and mailing tubes. Much computer software in diskette form is mailed to retail customers in envelopes and mailers lined with antistatic cushioning material.
A third application area for antistatic PU foam is in mats and pads for electrosensitive areas such as computer rooms. Footwear requiring antistatic soling is also a growing use for polyurethane foam and elastomers.
b. Classification of Electro-dissipative Materials:
The static-dissipative properties of materials such as PU foam are classified as follows:
Surface and Volume Resistivity
This is the broad preliminary separation of materials into three classifications based on ASTM Test Method D-257. Typically a measurement is made on the material to determine surface resistivity, and then the volume resistivity can be calculated knowing the thickness of the sample tested. Volume resistivity also can be measured directly using ASTM D-991.
"Antistatic" is defined within the industry with respect to ASTM Test Method D-257. Materials are classified as follows:
______________________________________ Surface Resistivity Category (in Ohms/square) ______________________________________ Conductive &lt;1 .times. 10.sup.3 Dissipative &lt;1 .times. 10.sup.12 Insulative &gt;1 .times. 10.sup.12 ______________________________________
This is the first broad classification of electrical properties. The D-257 classification has essentially refined and replaced the previously used "Triboelectric Series" as described in the article by Al Lerner entitled "A New Additive for Electrostatic Discharge Control in Foams and Elastomers", pages 31-34, Journal of Cellular Plastics (1985).
For static-sensitive packaging purposes, the interest is in producing PU foam that falls within the "dissipative" category. "Dissipative" polyurethane foam with surface resistivities of less than 1.times.10.sup.12 ohms/sq. can be produced by the addition of an antistatic additive to the formulation.
By contrast, to reach the "conductive" category usually requires a post-treatment applied by dipping or spraying a coating on the foam. In comparison, typical unmodified PU foam exhibits a surface resistivity value of &gt;1.times.10.sup.17 ohms/sq.
Static Decay Time
After the initial classification as "dissipative", the material is tested for Static Decay Time. This is defined as the amount of time it takes to dissipate a given electrical charge, with the test conducted according to Federal Test Method Standard 101, Method 4046 ("Electrostatic Properties of Materials"). While various specifications differ on the static delay time they require, most military and commercial specifications call for a value of less than 2 seconds.
Other Classifications
After a foam has passed the Surface Resistivity and Static Decay Time requirements, various military and commercial specifications call for additional requirements. Some of these specifications are:
Color: The industry standard colors are pink and charcoal gray. Typically, the pink color denotes foams with densities of around 1.3 pcf; the charcoal gray denotes 1.8 pcf.
Contact Corrosivity: This is a determination of how corrosive the foam is when placed in contact with another material. Obviously, the packaging material should not cause corrosion of the material it is intended to protect. Corrosivity is a function of what types of additives the foam contains, in particular the presence of certain amines and chloride-producing compounds (such as flame retardants). This test is defined by Federal Test Method Standard 101C.3005.
Dynamic Cushioning: This is required by certain military specifications and requires the plotting of graphs of "Peak Acceleration" versus "Static Stress" for varying thicknesses of foam. This test is required by the more sophisticated purchasers of packaging foams.
Others: Some specifications also include requirements for air flow, tensile, tear, elongation, density and firmness, all of the normal PU foam physical tests.
c. Prior Art Antistatic Additives:
Prior art antistatic additive formulations for PU were based on amine-based quaternary ammonium compounds. Such amine-based preparations are disfavored in PU foams used for electronic packaging applications because amines exhibit moderate to high levels of contact corrosivity (defined above) which can cause damage to the product itself.
The prior art amine-based additives are also migratory, and move to the surface of the PU foam as humidity decreases. Although such migration affords the best static protection at the surface of the material, the migration exacerbates the contact corrosivity problem. Another serious drawback to amine-based materials is that they affect the catalysis of the polyurethane reaction used to produce PU foam, and are therefore somewhat unpredictable in their actions from production run to production run. They also tend to soften the foam. As a result, amine-based preparations are not now widely used in PU foams.
Another prior art additive is disclosed in U.S. Pat. No. 4,618,630, issued Oct. 21, 1986 to Knobel et al., and assigned at issue to Dow Chemical Co. (the Dow additive). The Dow additive is an organic boron composition that is non-migratory, non-corrosive and chemically inert with respect to the polyurethane reaction. The Dow product has a relatively high cost, and a high use level (8-10 parts per hundred parts polyol) required to achieve acceptable antistatic performance.
The Dow additive also depends on and incorporates an "enhancer" compound as part of their additive, without which the Dow additive cannot achieve its antistatic test performance.